Bärbel MÜLLER-KARULIS 1, 2*, Timo ARULA 3, Maija BALODE 1, Kerli
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ACTA ICHTHYOLOGICA ET PISCATORIA (2013) 43 (2): 151–161 DOI: 10.3750/AIP2013.43.2.08 CHALLENGES AND OPPORTUNITIES OF LOCAL FISHERIES MANAGEMENT: PIKEPERCH, SANDER LUCIOPERCA (ACTINOPTERYGII: PERCIFORMES: PERCIDAE), IN PÄRNU BAY, NORTHERN GULF OF RIGA, BALTIC SEA Bärbel MÜLLER-KARULIS 1, 2* , Timo ARULA 3, Maija BALODE 1, Kerli LAUR 3, and Evald OJAVEER 3 1 Latvian Institute of Aquatic Ecology, 8 Daugavgrivas, LV-1007 Riga, Latvia 2 Baltic Nest Institute, Stockholm Marine Centre, Stockholm University, Stockholm, Sweden (present address) 3 Estonian Marine Institute of the University of Tartu, 14 Mäealuse, EE-12618 Tallinn, Estonia Müller-Karulis B., Arula T., Balode M., Laur K., Ojaveer E. 2013. Challenges and opportunities of local fisheries management: pikeperch, Sander lucioperca (Actinopterygii: Perciformes: Percidae), in Pärnu Bay, northern Gulf of Riga, Baltic Sea . Acta Ichthyol. Piscat. 43 (2): 151–161 . Background. A local stock of pikeperch, Sander lucioperca (Linnaeus, 1758 ), is a valuable fishery resource in Pärnu Bay, northern Gulf of Riga. Due to the late maturity of pikeperch in the bay, the stock is vulnerable to over - exploitation, whereas recruitment is highly dependent on climate factors. Because of its high market price, fish - ing pressure on the stock increased considerably in the 1990s, resulting in stock depression. To assist fishery management in Pärnu Bay we have simulated the effect of several scenarios (such as: fishing mortality, climate, and young-of-the-year food supply) on the pikeperch stock, catches, and revenues generated by the fishery . Materials and methods. A simulation system consisting of an age-structured population model, a recruitment com - ponent, and a forcing module of different climate, food supply to young-of-the-year, and fishing mortality scenarios was used to estimate the equilibrium stock size and catches under varying environmental conditions and exploitation strategies. Economic impacts of these scenarios were assessed based on the first selling price of pikeperch . Results. Under present climate and food supply for pikeperch young-of-the-year, the Pärnu Bay pikeperch stock is very sensitive to catches of immature fish. Warmer future climate conditions are likely to be beneficial for the stock, but also prey abundance for young-of-the-year influences potential stock sizes and catches. Compared to targeting its prey species, herring, which has a lower commercial value at current first selling prices, pikeperch acts as a “biomeliorator”, roughly doubling fisheries revenue in the bay . Conclusion. Flexible adaptive management methods should be used to estimate the yearly allowable pikeperch catch, taking into account food supply to young-of-the-year and climate conditions influencing recruitment . Keywords: Pärnu Bay, pikeperch, temperature, year class abundance, exploitation, fishing mortality, management INTRODUCTION After Estonia regained independence in 1991, new A local stock of pikeperch, Sander lucioperca export markets for pikeperch opened and privatization of (Linnaeus, 1758 ), adapted to brackish water conditions, is state farms offered local fishermen cheap access to fish - a valuable fishery resource in Pärnu Bay (Vetemaa et al. ing gear. With Estonia’s low average wages in the early 2000, 2006), a shallow bay in the Estonian exclusive eco - 1990s and high first-buyer prices for pikeperch, the aver - nomic zone in the Northern Gulf of Riga (Fig. 1). Despite age catch of pikeperch needed to provide the equivalent of fishing regulations, which were established in Estonia as an average monthly gross wage ranged from 43 kg in early as in the 1920s, this stock has been heavily exploit - 1993 to 240 kg in 1999 (Vetemaa et al. 2000, 2006). ed since the early 1930s, when large-scale export of this Consequently, pikeperch catches increased drastically and fish from Estonia started. After low catches in the 1960s its fishing mortality rose to the highest level ever record - new, biologically justified measures to protect pikeperch ed. Young age groups, including immature fish, became were implemented, such as protected areas and the use of an important part of the catches. This was caused by artificial spawning substrates to increase recruitment. Estonia’s weak legal instruments for regulating fisheries Catches then improved and reached 300 t in the early in the 1990s (Vetemaa et al. 2002) and by insufficient 1990s (Erm et al. 2003) . funding for enforcing the legal minimum size of * Correspondence: Dr Bärbel Müller-Karulis, Stockholms universitets Östersjöcentrum, Stockholms universitet, SE-106 91 Stockholm, Sweden, phone: +46 8 674 7574, fax: +46 8 16 37 18, e-mail: [email protected] . 152 Müller-Karulis et al. pikeperch in catches (Eero 2004). Pikeperch reach full the final bottleneck for recruitment to the stock maturity only at the age of 5 years, with females maturing (Lappalainen et al. 2002). later (4–5 years) than males (3–4 years), which makes the In addition to climatic fluctuations, eutrophication is stock sensitive to harvesting of immature fish (Erm et a major driving force of ecosystem changes in Pärnu Bay al. 2003). Because fishermen in the 1990s to a large extent and the adjacent Gulf of Riga. In the small, shallow and targeted young age groups before the end of their most rapid comparatively isolated Gulf of Riga surrounded by agricul - growth period, the profits of this fishery were much smaller tural land and industrial centres, eutrophication became evi - than would have been in the case of sustainable manage - dent earlier and was more pronounced than in the open Baltic ment. After 1997 landings decreased sharply (Eero 2004) Sea. Eutrophication increased rapidly from the 1960s and the fishery was temporarily closed in 2000–2003. through the 1980s, had a peak in the late 1980s, and Recruitment success determines the internal dynamics decreased slightly in the 1990s (Nehring et al. 1989, of the Pärnu Bay pikeperch stock (Eero 2004). The shal - Berzinsh 1995, Anonymous 1996, 2002). However, low bay (mean depth 5 m) covers about 700 km 2 and its pikeperch is well known to tolerate or even benefit from salinity ranges from PSU 1 through 5 (Kotta et al. 2009 eutrophication in freshwater systems (Garcia et al. 2006, and references therein). The climate is rather continental, Kangur et al. 2007) and Baltic coastal waters (Lehtonen et with ice covering the bay in winter and the water temper - al. 1996, Lappalainen et al. 2002, Sandström and Karås 2002). ature rising to 24ºC in summer (Arula et al. 2012). Therefore eutrophication most likely poses no direct Pikeperch spawns on sandy bottoms in May and June threat to pikeperch in Pärnu Bay. (Erm et al. 2003). The larvae are planktivorous, first con - Being the largest predator in the Pärnu Bay ecosystem, suming nauplii and young stages of Eurytemora affinis pikeperch converts the biomass of commercially unexploited and Acartia bifilosa (Copepoda). At the end of July fish—gobies ( Pomatoschistus spp.); bleak, Alburnus albur - young-of-the-year (YOY) pikeperch become piscivorous, nus (L., 1758); sticklebacks, Gasterosteus aculeatus L., at first preying chiefly on goby ( Pomatoschistus spp.) lar - 1758; roach, Rutilus rutilus (L.; 1758)—and species of mod - vae as well as Neomysis sp. and Corophium sp., and later erate value as herring, Clupea harengus L., 1758; mainly on juvenile gobies (Erm 1981a). and smelt, Osmerus eperlanus (L., 1758), into biomass of Strong year classes of pikeperch are produced when high commercial value and acts thus as a “biomeliorator”. spawning coincides with warm water and calm weather Therefore, sustainable management of pikeperch (Erm 1981a, b). Further, abundant food, especially goby resources is of high importance for the coastal population larvae and juvenile gobies, when the early juveniles switch around Pärnu Bay. To identify potential management to piscivory improves the growth and consequently sur - strategies for the Pärnu Bay pikeperch population we have vival during the following winter (Lappalainen et al. 2000). developed scenario simulations that address different The wintering conditions for the 0-group pikeperch, exploitation rates, weak or strict enforcement of the legal including winter severity, ice cover duration, abundance size limit, varying prey availability to young-of-the-year, of predators, and fishing intensity in Pärnu Bay, constitute and the effect of climate change on the stock dynamics. Fig. 1. Location of Pärnu Bay (crosshatched area) in the northern part of the Gulf of Riga, Baltic Sea Pikeperch management in Pärnu Bay, northern Gulf of Riga 153 MATERIALS AND METHODS the Pärnu Bay pikeperch stock (Eero 2004) as calibration To estimate stock sizes and catches of Pärnu Bay data, the final model explained 63.5% of the data deviance 2 pikeperch in equilibrium with different management (R adj = 0.48). All four variables contributed notably to the strategies, exploitation rates, as well as climate and prey explanatory power of the model and were significant at abundance scenarios, we have designed a simulation sys - 0.02 < P ≤ 0.10. Compared to models using only three tem with an age-structured population model for the explanatory variables, each variable improved the explained pikeperch stock, driven by fishing mortality ( F) and envi - deviance by 15.1 percentage points (goby abundance) ronmentally dependent recruitment, as core component. through 27 percentage points (summer temperature). The population model was forced by fishing mortality